Almost twenty-three years ago, Crivello and Lam published the first paper describing the use of triarylsulfonium salts bearing anions of poor nucleophilic character as a novel class of efficient photoinitiators for cationic polymerization. Since this initial paper and due to the significant contributions of many other researchers, the field of photoinitiated cationic polymerization has expanded rapidly. Triarylsulfonium salt cationic photoinitiators are in widespread and increasing commercial use and are currently employed in such applications as printing inks, can and beverage coatings, adhesives, as well as in photo- and stereolithography, among many others.
The most common commercially available triarylsulfonium salt photoinitiators consist of a highly complex mixture of related arylsulfonium salts in which the primary components are shown below.

A two-step reaction sequence is used for the synthesis of these materials. In the first step, the condensation of benzene with sulfur monochloride is carried out in the presence of elemental chlorine with aluminum chloride as the catalyst. Thereafter, the resulting mixture of mono- and bissulfonium chlorides are subjected to a metathesis reaction in which the chloride anions are exchanged to provide the active photoinitiator mixture bearing an anion, MtXn−, of poor nucleophilic character. Typically, MtXn− represents a complex anion of the type: BF4−, PF6−, SbF6−AsF6−, or, (C6F5)4B−. The ratio of the different salts as well as the many other minor components obtained is largely determined by the stoichiometry of the reactants in the complex condensation reaction shown above as well as by the conditions under which it is carried out. However, in practice, the mixture of sulfonium salts shown above varies substantially from batch to batch and, since the photosenstivities of the triarylsulfonium salts are different, irreproducible and intolerable reactivity variations in many applications are observed. The marginal solubility of these same photoinitiators in many monomers and functional oligomers may also contribute to the irreproducibility of the photosensitivity in some systems. Many nonpolar monomers and functional oligomers fail to polymerize altogether due to the poor solubility of the photoinitiators. Lastly, and most importantly, it has been noted that the UV irradiation of the above photoinitiators results in the formation of benzene as one of the photolysis products. This takes place during the photolysis of the salts chiefly as a result of the homolytic cleavage of the sulfonium salts to form phenyl radicals and their subsequent hydrogen abstraction reactions. Due to the formation of traces of benzene, triarylsulfonium salt photoinitiators have been recently banned from use in Europe in those applications (e.g. printing inks) in which there is the possibility of food contact.
Despite the drawbacks cited above, triarylsulfonium photoinitiators possess many very attractive features that have played a strong role in their acceptance in many commercial applications. They are highly photosensitive and efficiently initiate very rapid vinyl and ring-opening cationic photopolymerizations on UV irradiation. Formulations containing these photoinitiators with monomers have extremely long shelf lives in the absence of UV light. Moreover, mixtures of monomers and triarylsulfonium salt photoinitiators can be heated to temperatures in excess of 120° C. without initiating polymerization. The excellent thermal latency displayed by triarylsulfonium salts is a very desirable property for all practical applications and greatly aids in both the clean up and in the reuse of photosensitive materials. Commercially available sulfonium salt photoinitiators have strong absorption bands in the short wavelength as well as the mid-region (305 nm) of the UV spectrum that is highly desirable since common mercury arc lamps as well as certain laser light sources have prominent emission bands in this region. As a result, many users have optimized their UV irradiation equipment to accommodate the absorption characteristics of these photoinitiators and it would be advantageous to find replacement photoinitiators with the same or a similar UV absorption response. They are nontoxic and have a low incidence of eye and skin irritation. Lastly, the above triarylsulfonium salt photoinitiators are inexpensive and are not easily replaced on a cost basis by other onium salt photoinitiators such as diaryliodonium salts.
The original Crivello paper mentioned above and subsequent papers disclosed 5-phenyl- and 5-(4-methylphenyl)-thianthrenium salts (Crivello, J. V. and Lam, J. H. W., Polymer Journal (Tokyo, Japan) (1985), 17(1), 73-83; Polymer Photochemistry (1982), 2(3), 219-26; and Journal of Polymer Science, Polymer Chemistry Edition (1980), 18(8), 2677-95). In addition, EP 580552, EP 869393, U.S. Pat. No. 5,731,364, U.S. Pat. No. 4,161,478, disclose use of various classes of thianthienium salts as photoinitiators or photoresists. However, problems associated with current photoinitiators were not recognized at that time. WO 03/008404, WO 03/002557 and U.S. 2003/0017415 disclose thianthrenium salts that lie outside the subject matter of the present invention.
There is, therefore, a need for a replacement for the present commercially available triarylsulfonium salt photoinitiators that preserves the same desirable features as triarylsulfonium salt photoinitiators, but avoids the problems outlined above.